1. Field of the Invention
The present invention relates to a network storage virtualizing method and a virtualizing device for viewing multiple network storage devices as one network storage device from the client.
2. Description of Related Art
Information systems of the related art utilize local connections for storing information in computers for processing. However, along with the advance made in network technology in recent years, rather than placing information for storage in computers, the practice of connecting to a network and storing information by accessing a network has become widespread. Storing information by connecting to a network is referred to as network storage.
Typical examples of network storage are the SAN storage and NAS (Network Attached Storage). The SAN storage uses SAN (Storage Area Network) and has high reliability and performance. However the cost of SAN storage is extremely high and is mainly used by business enterprises. The NAS on the other hand uses the IP network which is the most widespread. The performance NAS delivers is lower than SAN storage but the cost is low compared to SAN and it is also easier to use.
In recent years, instead of high-priced large scale methods, demands have been made to obtain performance and capacity by using multiple small-scale NAS units. However, this method has the problem that when the number of NAS units is increased, the cost for managing those units also greatly increases. Whereupon, even if multiple NAS are made to appear as one virtual NAS (unit) from a computer and new NAS are added and current (existing) NAS eliminated in an effort to limit system management costs, NAS virtualizing (single-file) technology that does not adversely affect the computer is still needed.
A number of methods have been developed for NAS virtualizing (single-file) technology of this kind. A method is disclosed in http://www.maxtor.com/products/maxattach/default.htm for example, wherein a control server called a primary storage (unit) that also functions as a network storage unit manages the file positioning information in one batch. In this method, multiple network storage units determined by attributes of the storage unit such as the time the file is accessed, are made to appear as one virtual network storage unit. Newly generated files are stored in primary storage and files not accessed for a certain period of time are then shifted to secondary storage. When the primary storage receives a request from a computer to access a file, and that file currently resides in secondary storage; that file is accessed (while) in secondary storage so that only one network storage unit is viewed as seen from the computer.
In another method disclosed in, The Zebra Striped Network File System, Hartman et. al., ACM Transactions on Computer System, vol. 13, No. 3, 1995, pp. 274-310, a file is divided into file fragments and they are distributed among multiple network storage units. A centralized control server is installed to monitor in what sequence and what storage unit the file is stored in, so that multiple network storage units are virtually shown as one network storage unit. By using this striping method on one file in network storage areas, even when the disk size of the file is extremely large, data can be uniformly written into each network storage unit.
Another method was disclosed, DiFFS: A Scalable Distributed File System, Christos Karamanolis et. al., HP Laboratories Palo Alto, HPL-2001-19, Jan. 24, 2001 wherein, in contrast to the above mentioned centralized server method, a de-centralized server method is utilized to show multiple network storage units virtually as on network storage unit. In this method, the destination network storage unit of the file is stored within a directory entry that controls the directory and files, and this directory entry is dispersed and stored in network storage units.
Technology disclosed in U.S. Pat. No. 6,029,168 describes the above non-centralized server method along with a method for dividing upon file into file fragments and distributing them among multiple network storage units. In this method, each network storage unit has small information of file fragments and its locations.
The above methods; http://www.maxtor.com/products/maxattach/default.htm and DIFFS: A Scalable Distributed File System differed in that file management information was held by a centralized means or a dicentralized means, however information on each file was held as destination network storage information. The above Zebra Striped Network File system method and the method in U.S. Pat. No. 6,029,168 also held fragments (pieces) of the file as destination network storage information. When an individual file or file fragments are held in this way as destination network storage information, the management information greatly expands in proportion to the number of files, and this management information places a strain on the data storage area of the network storage unit.
Further, during structural changes such as when the destination network storage (unit) for the file is changed at times such as when new network storage is added or deleted from the multiple network storage units virtualized into one view; the destination network storage (unit) information must be rewritten for all files at the storage destination that were changed. This task of rewriting information has effects on performance when these structural changes are made.